On Tue 12-12-23 21:16:16, Baokun Li wrote: > On 2023/12/12 20:41, Jan Kara wrote: > > On Tue 12-12-23 17:36:34, Baokun Li wrote: > > > The following concurrency may cause the data read to be inconsistent with > > > the data on disk: > > > > > > cpu1 cpu2 > > > ------------------------------|------------------------------ > > > // Buffered write 2048 from 0 > > > ext4_buffered_write_iter > > > generic_perform_write > > > copy_page_from_iter_atomic > > > ext4_da_write_end > > > ext4_da_do_write_end > > > block_write_end > > > __block_commit_write > > > folio_mark_uptodate > > > // Buffered read 4096 from 0 smp_wmb() > > > ext4_file_read_iter set_bit(PG_uptodate, folio_flags) > > > generic_file_read_iter i_size_write // 2048 > > > filemap_read unlock_page(page) > > > filemap_get_pages > > > filemap_get_read_batch > > > folio_test_uptodate(folio) > > > ret = test_bit(PG_uptodate, folio_flags) > > > if (ret) > > > smp_rmb(); > > > // Ensure that the data in page 0-2048 is up-to-date. > > > > > > // New buffered write 2048 from 2048 > > > ext4_buffered_write_iter > > > generic_perform_write > > > copy_page_from_iter_atomic > > > ext4_da_write_end > > > ext4_da_do_write_end > > > block_write_end > > > __block_commit_write > > > folio_mark_uptodate > > > smp_wmb() > > > set_bit(PG_uptodate, folio_flags) > > > i_size_write // 4096 > > > unlock_page(page) > > > > > > isize = i_size_read(inode) // 4096 > > > // Read the latest isize 4096, but without smp_rmb(), there may be > > > // Load-Load disorder resulting in the data in the 2048-4096 range > > > // in the page is not up-to-date. > > > copy_page_to_iter > > > // copyout 4096 > > > > > > In the concurrency above, we read the updated i_size, but there is no read > > > barrier to ensure that the data in the page is the same as the i_size at > > > this point, so we may copy the unsynchronized page out. Hence adding the > > > missing read memory barrier to fix this. > > > > > > This is a Load-Load reordering issue, which only occurs on some weak > > > mem-ordering architectures (e.g. ARM64, ALPHA), but not on strong > > > mem-ordering architectures (e.g. X86). And theoretically the problem > > AFAIK x86 can also reorder loads vs loads so the problem can in theory > > happen on x86 as well. > > According to what I read in the /perfbook /at the link below, > > Loads Reordered After Loads does not happen on x86. > > pdf sheet 562 corresponds to page 550, > > Table 15.5: Summary of Memory Ordering > > https://mirrors.edge.kernel.org/pub/linux/kernel/people/paulmck/perfbook/perfbook-1c.2023.06.11a.pdf Indeed. I stand corrected! Thanks for the link. Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR
